The present invention relates to an acceleration/deceleration device employed in hydraulic open circuit type actuating circuits and particularly to ones which are used for hydraulic actuating circuits that require an appropriate and shockless acceleration/deceleration control.
No matter whether it is rotary or linear motion, controlled shockless acceleration/deceleration of a load has been considered very difficult to achieve to date. In particular, the most common mechanisms tried so far for controlling large loads, or loads having large inertia have been designed so as to minimize shock occurring in starting or stopping the load by enabling the flow passage area to change in proportion to the stroke of the spool of a direction switching valve for pilot use installed in a pilot passage circuit of the amplification valve. In this case, the pilot flow rate in the circuit is controlled by adjusting the spool stroke so as to change the flow passage area of the said amplification valve gradually and thus the main flow rate gradually increases or decreases in proportion to the said pilot flow rate. Still, this mechanism does not give satisfactory results. The reason is that since the control flow rate changes with supply pressure or load pressure because of throttle control mechanisms being used in the above cases, and unless the direction switching valve for pilot use is controlled very skillfully, a high pressure, is exerted on the actuator simultaneously with acceleration, or a high pressure is produced in the actuator simultaneously with deceleration as the return circuit is over-reduced, shock is imparted to the load. Even though the generation of such high pressure as mentioned above may be avoided by means of various compensatory adjusting mechanisms, the adjustment becomes complex and troublesome. In view of characteristics of throttle control, therefore, it will be difficult, without employing some other means, to achieve satisfactory control while maintaining an appropriate acceleration/deceleration curve. In this case, deviations occur from the acceleration/deceleration curve due to the changes in fluid temperature and pressure.
In cases where satisfactory control characteristics could not easily be obtained by simple means, closed circuits have been commonly adopted in the past, for both motor circuits and cylinder circuits, and variable capacity type pumps have also been adopted so that the delivery rate may be gradually increased during acceleration and may be gradually reduced by regenerative control methods or the like during deceleration, thus achieving smooth acceleration/deceleration control. According to the said mechanisms, it is feasible to prevent shocks from occurring during acceleration/deceleration only if attention is paid to the operation for changing the pump delivery rate during acceleration/deceleration of the load. The most noticeable defect of mechanisms incorporating such variable capacity type pumps, however, is that the pumps must respond each of the control objectives and that simultaneous operation cannot be achieved in parallel circuits for acceleration/deceleration control. In addition, if these circuits are different from each other in maximum velocity, it becomes very tedious work to control each of them by means of a pump section.